Yeast has been used in the manufacture of bread probably from prehistoric times, for certainly it was known to the Egyptians two thousand years before the Christian era. Its claims to recognition as an almost essential item in bread-making are therefore time-honoured, and until recent years no suspicion with reference to its hygienic position has ever been whispered. It could hardly be expected, however, that it could escape attention for all time, and so in common with practically every ingredient of the daily menu it has been impugned as a deleterious substance.

Is Yeast a Malign Agent? - It is categorically stated to be a putrefactive ferment, which, when used in bread, tends to continue the "putrefactive fermenting" process in the alimentary canal of the consumer, and is more or less productive of blood-poisoning, which may develop into and aggravate disease. A certain section of "food reformers," therefore, not altogether but mainly composed of vegetarians, is satisfied to subsist upon yeast-free bread, which, whatever its merits, lacks most of the attractions usually associated with the staff of life. The disciples of this sect maintain that "the digestive ferments and alcoholic ferments are opposite in character, the action of the former being a decomposition for the purpose of recomposition, and of the latter, a breaking up into ultimate products which are then suitable only for resolution as manurial elements." They contend that it is a fallacy to suppose that yeast-raised dough, when cooked, has all its yeast activities destroyed, but certainly the article in the Lancet (April 10, 1909) on which they rely for support in no way corroborates them. The writer of this article, in drawing attention to the amount of alcohol in hot cross-buns, points out that each bun, weighing two ounces and a half, contains 7.2 grains, or .68 per cent., of alcohol, equal to one-third of a fluid ounce of light beer, so that twenty buns would contain as much alcohol as a tumblerful of beer measuring 8 ounces. The presence of the alcohol is unquestionably due to the fermentative action of the yeast, the tenacious character of the dough, and its external albuminous glaze preventing its dissipation by the heat during baking.

No reference whatever is made to the continued activity of the yeast, but, even if this had been established, it would have been quite unfair to infer that bread would betray the same deficiencies, because the influence of the heat acting on a less tenacious dough, with a more permeable covering, is quite sufficient to ensure the destruction of most of the yeast and the evaporation of any alcohol which may be formed. Hence well-baked bread never displays on analysis anything but the merest traces of alcohol, to which even the most bigoted teetotaller need make no objection.

The virtues of this bread, however, do not terminate with the exclusion of yeast, for, like its allied products, such as cakes, biscuits, etc, it is salt-free, nor is any chemical substance whatever permitted to be added during its manufacture. Furthermore, nothing but the entire wheat grain, free from husks, and ground so finely that it is incapable of irritating the mucous membrane of the alimentary canal, is used. And to complete the list of its good qualities, it is mixed with distilled water.

The Baking Of Bread

It is impossible to appreciate too highly the extreme solicitude on the part of the manufacturers that nothing but the purest of materials should be employed in their products, and they deserve a high meed of praise for their spirited effort in the service of the public, but, apart from the performance of this important duty, it is questionable whether their action is not founded upon a misapprehension. In order that we may form a true estimate of their claims, it will be necessary for us to glance for a moment at the problem of bread-baking. Flour cannot be eaten uncooked.

To utilise it as food it must be cooked, and for this purpose all that is necessary is to mix it with water, mould it into a definite shape, and bake it. This will be recognised as ship's biscuit, a substance difficult to masticate and hence to digest. When spread into thin flat cakes, usually perforated, it is known as unleavened bread, and in this form is more easily digested, because more capable of being thoroughly masticated.

For the purpose of making bread it is essential that the mixture of flour and water, which is a tenacious viscid mass, should be interpenetrated by a gas of some kind, and it was discovered that this could be effected by mixing the dough with yeast cells and setting it aside in a warm place. Like many other micro-organisms, yeast cells are practically ubiquitous and capable of floating about in the atmosphere, looking about for a suitable soil upon which they may settle and grow. As this is usually a saccharine medium, the yeasts are generally known as saccharomycetes, and whenever they get a chance they develop with remarkable rapidity and produce a number of chemical changes, the most important amongst them being fermentation. When the yeast cells deposit themselves in dough left exposed to the air, bacteria of one kind or another likewise gain admittance at the same time. Hence both grow in the mixture of flour and water, extracting their nutriment therefrom, the yeast subsisting upon sugar, throwing off carbonic acid and alcohol as by-products, the bacteria in the same manner producing lactic and acetic acids. This mixture was called leaven, and its product leavened bread, a sour tasting and smelling substance with various drawbacks, quite corroborating many of the allegations made by the yeast-free advocates.

But it is now abundantly possible to obtain pure yeast prepared in a special way and purified by repeated washings, a much more active, agreeable, and wholesome substance than the old-fashioned leaven. Under certain circumstances, which need not here be specified, this yeast attacks with avidity the small proportion of sugar in the mixture of flour and water and multiplies with great rapidity, each molecule of sugar being converted into two molecules of alcohol and carbonic acid gas respectively. This gas fills up the dough with bubbles, which make it light and spongy, and when baked in an oven for one hour and a half at a temperature of 450° E. the bubbles of gas expand still further, the fermentation ceases because the yeast cells are killed, and bread is formed. In addition to killing the yeast cells, the excessive heat volatilises 71.2 per cent, of the fats, and about 8 per cent. of the starch on the outside is rendered soluble or converted into dextrin.

A large percentage of the carbonic acid and also of the alcohol is driven off, although quite 16 grains of the latter is contained in each four-pound loaf. A certain amount of the flour, therefore, has been wasted, and, besides the fat already mentioned, it is computed that 1.3 per cent. of the protein and 3.2 of the carbohydrates, or altogether 5 per cent. of the total caloric value, has been lost.

It is perfectly justifiable to promote efforts for the prevention of this loss of nutritive material, and besides the method under discussion, two others have been suggested. In the one case, air is forced under pressure into the dough, the bread, thus called aerated, being baked thereafter in the usual way. The result is quite successful, but for some reason - probably a somewhat insipid, uncooked taste - this bread has never been very much in favour. In the other, a baking-powder, consisting of an acid and alkaline carbonate, generates carbonic acid gas when water is added, and if thorough admixture with the dough has taken place, a perfectly sufficient aeration of the dough ensues. The objection to this method consists in the gratuitous importation of chemical substances, the daily consumption of which may in the long run be prejudicial to the best interests of the body.

There was a delicious comestible called a soda scone, of which I was accustomed to partake freely in my boyhood's days in Scotland, and I am not altogether sure that it did not aggravate a dyspeptic condition due to hyperacidity from which I then suffered much. Its peculiarity resided in the fact that it was home-made, the flour being mixed with milk and water, or skim milk alone, and baking-powder - consisting of an impromptu combination of tartaric acid and an excess of bicarbonate of soda - the product being rolled out into circular cakes and baked on a gridiron or "girdle" (griddle), as it is termed in the North. The excess of alkali was sufficiently strong to be tasted, and the general impression is - although it is hardly corroborated by Pavlov - that small doses of alkali, whatever their transient effective powers of neutralisation, only stimulate a further outflow of acid. In any case, bread made in this way is not at all popular, and whether from laziness or preference the public demand baker's bread.

Nor is this at all surprising, for from every point of view, despite the serious indictment of the yeast-free sect, the normal commodity or article is to be preferred. Where pure yeast is employed, or even where there has only been a negligible contamination with bacteria, the baking of bread should destroy both bacteria and yeasts; but unless the full conditions be complied with, and especially unless a sufficiently high temperature be applied for a sufficiently long space of time, the moist interior may not be reached effectively, and some of the micro-organisms may not be killed. It is quite possible to obtain yeast cells from the interior of. what is termed slack-baked bread, which will grow with a vigour competent to raise fresh dough. Bread should always be thoroughly baked, and when the process is completed, and it is removed from the oven, it should be cooled as quickly as possible. The practice customary among some domestic cooks of covering the freshly baked loaf with a thick cloth to retain the steam, in order that it may soften the crust, favours the continuance of the germ growth within the loaf.

The Beneficial Effects Of Yeast

No harm need, therefore, be anticipated in an ordinary way from the dead yeast cells in properly baked bread, and even although a few live ones are encountered, they are more than likely to have a beneficial influence. Combe concludes his dissertation upon yeasts with this statement: "The yeasts are microbicidal, they attenuate the toxins, augment the number of phagocytes, and favour intestinal peristalsis - qualities which must render them useful and valuable in exaggerated intestinal putrefaction and consequently in digestive auto-intoxication." Brewer's yeast, he declares, is an "energetic antiputrefactive medicine," and is consequently of great value in gastric dilatation with exaggerated fermentations where there is not prolonged retention. It is likewise of value in acne, furunculosis, urticaria, etc. - diseases which are in some measure associated with the absorption of toxins from the alimentary canal. In any case, as the yeast cell is ubiquitous, even the consumer of yeast-free bread must be exposed to its action, and so would be subjected to the same risks of "blood poisoning which may develop into and aggravate disease" - quite a gratuitous declaration after the distinguished authority I have just quoted.